1. Field of the Inventions
The present inventions relate to method and apparatus for detecting the locations of and efficiently removing defects in a thin film solar cell such as a Group IBIIIAVIA compound thin film solar cell fabricated on a flexible foil substrate to improve its efficiency.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is crystalline silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since the early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS-type, or CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded high conversion efficiencies. Specifically, Cu(In,Ga)Se2 or CIGS absorbers have been used to demonstrate 19.9% efficient solar cells. In summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated over a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown on a conductive layer 13 or contact layer, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. The substrate 11 and the conductive layer 13 form a base 13A on which the absorber film 12 is formed. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use the conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO etc. stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. The transparent layer 14 is sometimes referred to as the window layer. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1.
The conversion efficiency of a thin film solar cell depends on many fundamental factors, such as the bandgap value and electronic and optical quality of the absorber layer, the quality of the window layer, the quality of the rectifying junction, etc. A common practical problem associated with manufacturing thin film devices, however, is the inadvertent introduction of defects into the device structure. Since the total thickness of the electrically active layers of thin film solar cells is in the range of 0.5-5 micrometers, these devices are highly sensitive to defectivity. Even the micron size defects may influence their illuminated I-V characteristics. There may be different types of defects in thin film solar cell structures. Some of these defects may be only morphological in nature and are not electrically active. Other defects may be electrically active and may negatively impact the performance of the device. Shunting defects, for example, may introduce a shunting path through which the electrical current of the device may leak Such shunting defects lower the fill factor, the voltage and the conversion efficiency of the solar cells, and therefore they need to be minimized, eliminated or passivated. Detection and passivation of harmful defects improves the yield of thin film solar cell processing and therefore may be critical for low cost, high efficiency thin film solar cell manufacturing.
Prior work in eliminating shunting defects in solar cells includes work by Nostrand et al. (U.S. Pat. No. 4,166,918) who used an approach to bias the cell and heat up the shunts that carry a high current. A cermet material was incorporated into the cell stack which preferentially formed insulators at the shunt positions during the bias due to local heating. Izu et al. (U.S. Pat. No. 4,451,970) scanned the surface of the solar cell with a contacting liquid bead which electrochemically etched or anodized the shorting regions. The etched regions could then be filled with a dielectric. This technique may be applicable for the amorphous Si type solar cells. However, etching or anodizing of CIGS type compound materials leaves behind conductive residues comprising metallic species of Cu, In or Ga at the etched location that actually may make shorting even worse than before etching. Phillips et al. (U.S. Pat. No. 4,640,002) used a Laser Beam Induced Current (LBIC) technique to locate shorting defects on solar cell structures and then burned the shorts out by using a high power laser beam. A similar approach is recently used in US Patent Application 2007/0227586. Hjalmar et al. (U.S. Pat. No. 6,750,662) scanned the surface of Si solar cells with a voltage point probe and applied a voltage or light bias (illumination) detecting areas with shunts. This approach may work for thick crystalline solar cells, but would damage thin film devices. Glenn et al. (U.S. Pat. No. 6,225,640) used electroluminescence imaging on completed solar cells and removed detected defects chemically. Again such an approach is not applicable to flexible thin film devices such as CIGS cells, because as will be discussed later, defects in such thin film structures need to be detected and fixed before the solar cell is actually completed. Zapalac (US Patent Publication 2007/0227586) used laser scanning to determine shunts on finished solar cells and described ways of shunt removal by ablation or scribing.
As the brief review above shows, the importance of detection and removal of shunt defects in solar cells has been recognized for many years. Much of the work has concentrated on standard Si solar cells and techniques have been developed to detect shunts in finished devices. In thin film structures using CdTe, polycrystalline Si, amorphous Si, and CIGS absorber layers, the nature and chemical composition of the layers within the device structure are widely different, changing from a single element (Si), to more complex compounds such as a binary compound (CdTe), a ternary compound (CIS), a quaternary compound (CIGS), and a pentenary compound (CIGSS). Therefore, one defect removal method which may work for one device may not work for the other. The laser ablation method that is used for shunt removal, for example, is very successful for Si devices because Si can be easily ablated without leaving behind debris that would affect device performance. For CdTe, the process window for laser ablation is narrow because there is the possibility of formation of conductive debris comprising metallic Cd and/or Te at the location where laser ablation is performed. There is to date no successful laser ablation process for CTGS absorber materials because laser heating of this compound semiconductor leaves behind conductive phases comprising Cu, In, and Ga metals and/or Cu—Se phases. Such conductive phases introduce further shunts in the device structure at the laser ablated locations. Similarly, techniques using chemical etching of defect areas introduce problems for devices employing compound semiconductors such as CdTe and CIGS(S). In such compounds, chemical etching does not etch the material uniformly and leaves behind conductive residue.
Defects reduce the performance of the completed solar cell, and in particular the shunt resistance introduced by defects reduces the fill factor and thus the efficiency of the device. Some prior art methods used chemical approaches to etch away or anodize at least one of the transparent conductive layer, the absorber layer or the contact layer of the solar cell at the exact location of the defect with the goal of passivating the defect. In another approach a laser was used to ablate the defective region. Yet another technique applied a physical tool such as a scriber on the defect with the goal of physically eliminating it. Such approaches do not yield good results for thin film solar cell structures, especially for devices employing compound semiconductor absorber layers constructed on conductive foil substrates. For example, for a CIGS-type solar cell fabricated on a 25-100 μm thick metallic foil substrate, chemical etching or anodization methods do not work well because CIGS is made of Cu, In, Ga and Se and chemical or electrochemical etching of this compound material does not remove all these different materials at the same rate and leaves behind residues that may be conductive. Therefore, while removing a shorting defect, new shorts may be introduced in the device structure where the chemical or electrochemical etching process is performed. Furthermore, if the defect is under the grid pattern, etching techniques cannot be used because etching the grid which is a thick layer is not very practical. Mechanical processes that try to scratch away the defect may introduce even more defects, especially since the defect itself comprises highly conductive debris shorting the device. Physical scratching right on the defect actually smears such conductive debris and often makes the electrical shorting even worse in thin film solar cell structures.
An exemplary CIGS type solar cell, for example, typically has a 100-300 nm thick transparent conductive layer, a 50-100 nm thick buffer layer, a 1000-2000 nm thick absorber layer and a 200-500 nm thick contact layer. The substrate is typically 25-100 μm thick and the grid pattern has a thickness in the 5-50 μm range. In such thin and flexible device structures scribing over the defect with a mechanical tool peels off and damages the various device layers mentioned above at the vicinity of the defect which already has a shorting path for the electrical current, and also damages the metallic substrate which is flexible and pliable. Such damage from the conducting parts of the solar cell may create conductive debris shorting the top surface of the device to the substrate, the debris originating from the damaged substrate region, the contact layer as well as the transparent conductive layer and especially the portion of the grid pattern damaged by the scribing tool. Laser approaches used to remove defects from standard solar cells also do not work well for foil based thin film devices such as CIGS type devices. First of all, adjustment of the laser power to remove only the top transparent conductive layer or the grid pattern at the defect region is very difficult, and sometimes impossible. Laser beam heating of the grid pattern and/or the metallic substrate; may cause local melting of the metal substrate and cause new shorting defects. Laser removal of CIGS itself may create conductive debris around the removal area comprising metallic species such as Cu, In, and Ga. Such conductive debris is a source of new shorting defects in the device structure. Especially the most serious shorting defects which are under the grid pattern may not be removed by laser processes.
Therefore, there is a need to develop defect detection and passivation approaches that are specifically suited for CIGS-type thin film device structures on flexible foils.